Increasing attendance at 4.20 day events across America reflects public support for relaxing laws.

America’s greetings card industry makes hundreds of millions of dollars from celebrating such events as birthdays, Mother’s Day, bosses’ day and a day to mark the labours of the nation’s secretaries. But it is not yet easy to find a card to wish anyone “happy 4.20 day” – despite the fact the day is often very happy indeed.

Yet in spite of the lack of support from the greetings card industry tens of thousands of merry pot smokers across the US last week joined the annual gatherings, on 20 April, where the puffing of marijuana is openly and enthusiastically celebrated.

This year the events were especially large and public because America is slowly waking up to the fact that it is effectively legalising the drug across vast swaths of the country. In San Francisco, New Hampshire and even Alaska groups of people met to smoke cannabis and mark 4.20 day wreathed in a thick cloud of pungent smoke. In Denver, Colorado, a rally in front of the state Capitol attracted thousands of people. In nearby Boulder, an estimated 15,000 people all exhaled at the same time – 4.20pm, of course – in a scene that must have made the Colorado mountain town seem suddenly oddly foggy.

Typically, perhaps, there is some confusion as to how the 4.20 holiday got its name. Most signs point to a youthful shorthand for pot-smoking linked to the time when it was safe to light up a joint after the end of the school day. It might have begun somewhere in northern California. No one is quite sure. But what is definitely not in doubt is the steady legalisation of pot smoking under the guise of liberalising laws governing the use of medical marijuana. Now 14 states, including the nation’s largest, California, allow the use of medical marijuana by their citizens. More are likely to follow.

Contrary to the fevered warnings of anti-drug pundits, there has been no collapse of society or vast jump in drug-related crime. In California, where it is now easy to buy marijuana at a variety of shops and stores, there will be a November referendum on whether to legalise it properly – and indeed tax it. Two polls last week both showed a clear majority of the state’s citizens were in favour.

So this time next year – in California at least – perhaps even Hallmark will make a “happy 4.20 day” card.